Archive for April 2012

MOSQUITOES   Leave a comment

DEFINITION: any of a large family (Culicidae) of two-winged dipteran insects, the females of which have skin-piercing mouth parts used to extract blood from animals, including humans: some varieties are carriers of certain diseases, as malaria and yellow fever.

More than just annoying insects, some mosquitoes are responsible for transmitting diseases that can result in serious illness and even death. Mosquitoes were once viewed merely as a nuisance because of the itching and irritation that resulted from their bites. In the early 1900s, however, they were recognized as carriers of yellow fever, malaria, and other diseases.

The mosquito is in the family Culicidae and belongs to the same order of insects as flies and gnats the order Diptera and has the same anatomical structure. Its soft body is covered by an exoskeleton (an external supportive covering) and divided into three parts: the head, thorax, and abdomen. It has two narrow wings and a pair of knob-like structures, known as halters, that are present in place of a second pair of wings. Unlike other Diptera, the wings of the mosquito have tiny scales on the veins.

The mosquito’s head is rounded and supported by a slender neck. It has large compound eyes, complex mouth parts, and two antennae, usually divided into 15 segments. The antennae of the male are more feathery in appearance than those of the female. The major body segment behind the head is called the thorax, to which the wings and six legs are attached. The legs are long, slender, and segmented. The final segment of the mosquito’s body is the soft, cylindrical abdomen. It has ten segments, the last ones bearing the openings for the anus and reproductive organs.

Proboscis, snout, trunk, or other tubular organ projecting from the head of an animal.

The most dangerous parts of a mosquito’s anatomy are the female’s mouth parts These are modified into a proboscis for piercing and sucking. The proboscis looks like a single thin tube and is straight in most species. It actually consists of a sheath (the labium) that encloses saw-tipped daggers (the mandibles and maxillae), an injection tube (the hypopharynx), and a sucking tube (formed by closing the labium against the hypopharynx). The construction of the proboscis is ideal for removing blood from beneath the skin of animals. The mouth parts of the male mosquito are modified for feeding on plant juices; male mosquitoes do not bite.

Mosquito Bites

Not all species of mosquitoes suck blood. However in some species a blood meal by the female is essential to the reproductive cycle. In most species the females, like the males, suck nectar and other juices from plants for nourishment. The bloodsucking species feed primarily on mammals or birds, though some mosquitoes will feed on reptiles and amphibians. Some species are particular in their choice of host species, whereas others appear to be less selective. The feeding periods of many types of mosquitoes are restricted to particular times of the day or night.

To obtain a blood meal, a female mosquito selects a likely spot on her victim, brings her labium against it, and begins sawing through the skin with her mandibles and maxillae. Through her hypopharynx she injects saliva into the wound to prevent the blood from clotting so that it flows freely into her labro-hypopharyngeal tube. She then sucks up a supply of blood, stores it in her abdomen, and flies away.

The itching of a mosquito bite is caused primarily by the saliva that has been injected. If the mosquito completes her withdrawal of blood before being driven away, much of the saliva will be removed and the itching may be less severe.

Life Cycle and Habitats

A mosquito’s life cycle is one of complete metamorphosis it consists of four distinct stages: egg, larva, pupa, and adult though the pattern of development may vary between species. The female mosquito typically lays her eggs in standing water, where they float on the surface in a tiny cluster. The eggs may also be deposited singly or attached to vegetation, depending upon the species of mosquito. Some mosquitoes lay their eggs in the vicinity of water rather than directly in water, and the eggs develop when the area becomes flooded.

During warm weather the eggs develop into larvae within two or three days. Mosquito larvae are long, transparent, and constantly wriggling as they move up and down in a water column. They feed on organic matter, including small animals, bacteria, dead plant material, and algae. Some species feed on other mosquito larvae.

Pupa, quiescent stage between larva and adult in insect metamorphosis.

As the larvae grow, they periodically shed their skins (called moulting) in order to accommodate their larger bodies. Mosquito larvae normally moult four times. After the final moult the animal emerges as a pupa. The pupa has an enlarged anterior portion, composed of a head and thorax, and a curved, elongate abdomen. The pupa is aquatic but does not feed. Both the larvae and pupae of most species must come to the water’s surface to breathe. After two or three days the pupa develops into an adult, emerges from its pupal case, and flies away.

Mosquitoes vary in their courtship and mating habits. Many species mate while in flight. The males of some congregate in huge swarms, to which the females are then attracted. The humming sound made by mosquitoes is often a signal to attract mates.

In cooler temperate regions, adult mosquitoes hibernate, emerging in the spring to lay eggs. In some species mating occurs before the approach of winter and the males die, leaving only fertilized females. In others, eggs are laid in the fall and survive the winter without harm to hatch in the spring.

Mosquitoes are found almost everywhere in the world except open ocean areas, the most arid deserts, and the polar regions. Because of their dependence on water for development during their first stages of life, mosquitoes are most abundant in wet regions of the world. Nevertheless, of the more than 150 species of mosquitoes that inhabit the United States, many persist in arid regions of the South west Some species thrive in the extremely cold climates of Canada and Alaska, where vast swarms can sometimes be seen around some of the larger lakes and marshes.

Mosquitoes live in a wide variety of aquatic habitats. Besides lakes, ponds, and marshes, some mosquitoes lay their eggs in small depressions where water has collected temporarily. For example, many species use tree holes or fallen leaves, where water has accumulated after rains. In urban areas, common egg-laying sites for mosquitoes are empty containers that have collected water. Furthermore, mosquitoes are not restricted to fresh water for egg laying salt marshes are also a common habitat of many species.

1902: Cure for yellow fever. Walter Reed was a physician and bacteriologist in the service of the United States Army when he proved that yellow fever is transmitted by mosquito bites. Throughout the 19th century the general assumption was that yellow fever was transmitted by contact with such articles as clothing or bedding touched by someone who had the disease.

A Cuban doctor, Carlos Juan Finlay, theorized that the disease was carried by insects, but he had not been able to prove it. In 1896 an Italian scientist, Giuseppe Sanarelli, isolated the organism Bacillus icteroides from yellow fever patients. Reed, along with physicians James Carroll and Aristides Agramonte, was assigned the task of investigating the bacillus. At the same time, a yellow fever outbreak started in the American military garrison in Havana, Cuba. The three travelled there in the summer of 1900 and, by 1902, proved that mosquitoes were the carriers of the disease.

Shortly afterwards an insect extermination program was undertaken, and Havana was freed of yellow fever within 90 days. Colonel William Crawford Gorgas of the U.S. Army Medical Corps later used Reed’s techniques to rid Panama of yellow fever, making way for the construction of the Panama Canal.

Mosquitoes and Disease

Mosquito-transmitted diseases differ in their geographic distribution, specific causes and effects, and in the types of mosquitoes that transmit them. Yellow fever is caused by a virus that is transmitted primarily by the mosquito species Aedes aegypti, found in tropical and warm temperate regions of Africa and the Americas.

The primary mechanism of transfer of the yellow-fever virus (as well as other disease-causing organisms) is the mosquito bite specifically, when a mosquito bites an infected person and then bites a healthy one. The virus is thus passed from one person to another through the fluids from the mosquito’s mouth. The yellow-fever virus can also be present in other mammals, including monkeys, armadillos, and rodents, and a mosquito can transmit the disease to humans after biting an infected animal. Yellow fever attacks the liver, kidneys, and digestive tract, producing high fever and jaundice, a yellow skin colour from which the disease gets its name. More than half of the victims of yellow fever die within a few days. Those who recover are immune thereafter.

Malaria, disease consisting usually of successive chill, fever, and “intermission” or period of normality.

Malaria is another disease transmitted by mosquitoes. It is caused by microscopic protozoan parasites of the genus Plasmodium. The transmission of malaria is more complicated than that of yellow fever because the parasite must spend a portion of its life cycle inside a mosquito and the other part inside a human. (Yellow fever is dependent on the mosquito only as a transmitting agent.) Malaria is transmitted by mosquitoes in the genus Anopheles.

When an Anopheles mosquito bites a person infected with malaria, it may ingest blood that contains parasites in the sexually reproductive stage, called gametocytes. These gametocytes unite in the mosquito’s digestive tract and produce egg-like cells that burrow into the intestinal wall. They then hatch into free-swimming forms that travel to the mosquito’s salivary glands.

When the mosquito bites an uninfected human, the free-swimming parasites are transmitted to the victim through the mosquito’s saliva. These tiny parasites then enter the victim’s red blood cells and begin to divide to form new parasites. Eventually, the affected blood cells burst, and the parasites are released to enter new blood cells within the host and repeat the process of growth and division.

Within one to two weeks millions of these parasites are being released from burst blood cells, resulting in the characteristic symptoms of malaria: periodic chills and fever. Within ten days to two weeks after the initial infection, a new generation of sexually reproductive parasites develops in the blood of the victim. These parasites produce gametocytes, and victims can then infect any Anopheles mosquito that bites them. In this way the cycle of the disease is perpetuated.

In many areas of the world, including North America, mosquitoes of the genus Culex are transmitters of viral encephalitis (sleeping sickness) and other diseases. Dengue, or “break bone fever,” is a common tropical disease that results in muscular pains and eruptions of the skin. It is transmitted by Aedes and Anopheles mosquitoes.

Filariasis, disease caused by roundworms and transmitted by mosquitoes.

Roundworm, worm of the phylum Aschelminthes and the class Nematoda.

Filariasis, a disease that affects the lymph glands, is caused by parasitic roundworms and is transmitted by several different mosquito species in tropical regions.

Mosquito-transmitted diseases can be controlled through the elimination of mosquitoes or their egg-laying sites, medical treatment of victims, and prevention of mosquito bites through the use of insect repellent or protective clothing. As early as the 1700s South Americans recognized that quinine, an alkaloid obtained from the bark of the cinchona tree, alleviated the symptoms of malaria, though they did not know how the disease was transmitted.

In the late 1800s mosquitoes were implicated in the transmission of yellow fever in Cuba, and in the transmission of malaria in India. The United States Army initiated the first major effort to eradicate a mosquito-transmitted disease when it launched its campaign to quell the Cuban yellow-fever epidemic. Once the relationship between mosquitoes and yellow fever was understood, major projects were undertaken to eliminate the egg-laying sites of the Aedes mosquito. Similar measures were taken in malaria-infested areas of the world.

In addition to eliminating mosquito habitats, large-scale production began of chemical products that would kill mosquitoes or their eggs. Aerial sprays have been developed to kill adult mosquitoes. Toxic chemicals and oil products have been used in aquatic habitats to kill mosquito eggs, larvae, and pupae. Such chemicals must be used with caution because of their potentially damaging environmental effects. Mosquito bites can be prevented effectively with the use of a wide variety of insect repellents.

Assisted by J. Whitfield Gibbons, Senior Research Ecologist and Professor of Zoology, Savannah River Ecology Laboratory, University of Georgia.

Posted 2012/04/29 by Stelios in Education

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Brakes put on R20bn e tolling plan   Leave a comment

The South African public have spoken through the words of Judge Prinsloo.

The ruling that the contentious R20bn e-toll project in Gauteng has been halted was met by applause from the public gallery.

via Brakes put on R20bn e tolling plan.

Posted 2012/04/28 by Stelios in News

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SEWAGE DISPOSAL   Leave a comment

Perhaps no factor is more useful in the control of disease than the science of sewage disposal. It safeguards a community’s water supply by removing water-carried wastes including microscopic dissolved material, solid matter such as human waste, and harmful chemicals and bacteria.

Sewage is generally divided into two classes: domestic, or sanitary, sewage and industrial waste. Domestic waste water includes the used water of businesses and homes; industrial waste water is that discharged during industrial operations. Both the strength and volume of waste water may be markedly influenced by industrial wastes, which constitute about 80 percent of the sewage in the United States.

Sewage Treatment

Sewage systems collect waste water and treat it before discharging it back into the environment. These systems consist of intricate networks of underground conduits, or sewers, that convey the sewage through the treatment process to the point of disposal.

Sewage systems also handle the flow of rainwater, either separately or as part of a single system. Separate systems are generally preferable because, in single systems, heavy rainfall can overload treatment plants, with the result that untreated overflow can become a source of pollution. In separate systems, rainwater is often allowed to flow into streams untreated because it is assumed to be relatively clean.

Sewage is processed in three major steps, called primary, secondary, and tertiary treatments. Most areas do not use all three, and different areas use the treatments in different ways.

Primary treatment. The initial, and sometimes the only, method of cleaning waste water is primary treatment, which consists of removing floating chunks and fine particles of solid waste. The simplest form of primary treatment is a cesspool, now found primarily in rural areas. A cesspool is a big tank with a porous bottom and sides that lets the liquid waste water filter into the ground while holding the solid waste. Periodically the tank must be cleaned; the solid matter, called sludge, is sometimes used for fertilizer or landfill. Septic systems are somewhat similar, though the tank is connected to a drainage field so that more waste can be dispersed over a wider area.

In larger communities, sewer water passes first through a screen, which filters out the larger debris. It then runs through a grit chamber, a long, shallow trough with a dip in the bottom that acts like a trap. As water moves through the trough, small, hard materials in the water drift down to the bottom and fall into the trap. Grease floats to the surface and is skimmed off. The trap, like a cesspool, is periodically scraped clean.

After going through the screen and grit chamber, the sewage still contains small suspended solids about 1 ton per million gallons (3,790,000 litres) of waste water To remove some of these, the sewage is trickled into a sedimentation tank, or settling basin. The water enters through a pipe, then circulates slowly while the suspended particles settle to the floor. The top layer of water continually runs out through exit holes.

The sludge from sedimentation tanks may be sent through a tank called a digester, where bacteria digest it, producing carbon dioxide and methane gas and other by-products. Any combustible gases may then be collected and used to heat the digestion tanks and buildings and to fuel gas engines in the plant. The sludge may also be buried or dumped as landfill, burned, or dried in sludge drying beds for use as fertilizer.

Primary treatment removes about half of the suspended solids and bacteria in sewage, and about 30 percent of the organic wastes. Sometimes chlorine gas is added to the effluent (the liquid remaining after sedimentation) to kill most of the remaining bacteria. Some cities use chemicals that coagulate some of the solids into particles of a size and weight that will settle, so that they can be separated in a settling tank. The use of chemicals makes it possible to remove 80 to 90 percent of the suspended solids.

Secondary treatment. Today, large cities are usually required to put their wastes through both primary and secondary treatment because primary treatment alone removes so little organic material. Secondary treatment uses aerobic, or oxygen-breathing, bacteria to decompose organic wastes. The main object is to put the waste water in contact with as many bacteria as possible while keeping it aerated so that the bacteria have an adequate supply of dissolved oxygen.

One of the most common secondary treatments of this type is the activated-sludge method, so called because it uses sludge that is activated, or teeming with micro-organisms After going through primary treatment, the sewage is put into the activated-sludge tank, where it is aerated by pumps or blasts of compressed air. The compounds produced by the bacteria remain mostly suspended in the water and flow out with it into a secondary sedimentation tank.

The sludge from the bottom of the tank is handled in much the same way as the sludge from the primary sedimentation tank, except that about a quarter of it is recirculated back into the activated-sludge tank. This recirculation serves to seed the activated-sludge tank with fresh bacteria. The activated-sludge method permits almost any desired degree of treatment by varying the period of aeration. It removes about 95 percent of bacteria and more than 90 percent of suspended solids and organic matter.

Another method of secondary treatment is the trickling-filter method. Generally, rotating arms slowly spray the sewage over a shallow circular tank containing a layer of gravel or crushed rock. The rocks are covered with a slimy coating of micro-organisms that break down the organic wastes in the sewage. After this process, as in the activated-sludge method, the water that has been filtered is passed into a secondary sedimentation tank for removal of organic matter that has sloughed off from the stones of the filter. Trickling filters, together with primary treatment and final sedimentation, will remove most suspended solids.

Tertiary treatment. Waste water that has received primary and secondary treatment still contains dissolved materials that make it unsuitable for almost all uses except irrigation. Tertiary treatments, which depend largely on artificial chemical processes, are designed to remove these materials in order to make the effluent safer to discharge into waterways and safer for industry to use. A number of methods may be used, including radiation treatment, discharging the effluent into lagoons, and chlorination.

Sewage may also be passed through filters made of activated carbon, which consists of finely ground charcoal grains with rough, pitted surfaces that trap impurities. Alternatively, sewage may be strained through a screen made of tiny seashells called diatomaceous earth. The effluent may also be treated with chemicals that transform the dissolved organic material. Some chemical compounds, for example, combine with the nitrates in sewage to produce various salts. Such treatments are expensive, however, and are difficult to perform routinely.

History

The use of specially constructed sewers dates to the time of Babylon and ancient Greece, but only during the 19th and 20th centuries was the water-carriage sewage system adopted in the Western world. In these early systems, streams often served the dual purpose of sewage disposal and water supply, and hence there were frequent, disastrous epidemics of cholera, typhoid fever, and other water-borne diseases. The most effective methods of sewage treatment were not developed until the second quarter of the 20th century. Today, because of the greater amount of sewage from growing populations and industrial activity, there is an unprecedented quantity of legislation designed to control water pollution. As a result, scientists and engineers continually search for methods to further increase the levels of sewage treatment.

Posted 2012/04/21 by Stelios in Education

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GENETICS (Part 1 of 3)   Leave a comment

DEFINITION:1 the branch of biology that deals with heredity and variation in similar or related animals and plants 2 the genetic features or constitution of an individual, group, or kind.

April 28, 1994: Biological clock found in mice. Evidence for a so-called biological clock in mice was announced by scientists at Northwestern University in Illinois. It was the first time that a gene governing the daily cycle of waking and sleeping, called the circadian rhythm, had been found in mammals. Previously, genes governing circadian rhythms had been found only in fruit flies and bread mould The biological clock gene in mice was found on mouse chromosome number 5. The chromosomes of all living things hold the DNA, which determines the genetic make-up of each individual. Scientists hoped that this research would someday help them find a similar gene governing the biological clock in humans.

Why do human children resemble their parents? Why do the offspring of any species resemble their parents? Biologists have shown that the factors which cause such resemblances are passed on relatively unaltered from generation to generation by a process called heredity. Resemblances, they say, are transmitted by genes, cell units too tiny to be seen even with a microscope. The branch of biology that deals with genes is called genetics.

Through the ages men have speculated about heredity. In ancient Greece, for example, it was thought that the blood was in some way responsible for the transmission of hereditary traits, and the word “blood” is still often used to mean ancestry. Since the beginning of the 20th century, however, genes have been known to be the carriers of traits, though until the 1940s very little was known about them. Scientists recognized that genes were directly responsible for the characteristics of an organism and that genes were transmitted from parents to offspring. However, they had little idea of the gene structure and composition that made these actions possible.

By the 1950s scientists had learned a great deal about the chemistry of genes. Genes were found to be segments of certain complex molecules located in the cell nucleus. The molecules have the unique ability to duplicate themselves and, in so doing, to pass on body-building instructions to the next generation of a species.

THE ORIGINS OF MODERN GENETICS

Even before the beginnings of written history people were aware of some of the ways in which heredity takes place. The domesticated animals and plants of today are proof of this. Today’s domesticated horses, cattle, dogs, corn, wheat, and cotton differ greatly from their primitive, “wild” ancestors. They are products of the ancient breeders’ art, an art that included the proper selection of parents, well-controlled matings, and the careful choice of the best offspring to further improve a breed.

Early Theories of Heredity

Over the centuries more and more became known about the control of heredity for practical purposes. However, scientists remained baffled about the actual processes of trait transmission. All sorts of what proved to be erroneous explanations were advanced. In the 17th century, for example, a group of biologists called the ovists held that the ovaries of females contained the hereditary material and that the male sperm merely triggered embryonic development. Other scientists were of the opinion that tiny but fully formed creatures were present in the sperm.

Early in the 19th century the French biologist Jean Baptiste Lamarck suggested that traits and abilities acquired during the lifetime of an organism could be transmitted to future generations. This theory was termed “the inheritance of acquired characteristics.” Long before Lamarck, notions of this kind had led expectant mothers to practice the piano, gaze at beautiful pictures, or think “kind” thoughts in the hope that this would affect the character of their unborn children. For similar reasons, many breeders exposed plants and animals to the environmental conditions their breeding programs were intended to combat. Genetic discoveries in the mid-1800s proved Lamarck’s view to be mistaken.

1859: Darwin’s theory of evolution. A heated debate that continues to this day was sparked in 1859 with the publication of Charles Darwin’s ‘On the Origin of Species by Means of Natural Selection’. This work was immediately recognized by the scientific community as a landmark treatise on biology and evolution, but some Christians saw it as a threat to their theology.

Charles Darwin began his observations in December 1831 when, at age 22, he left England for South America aboard the exploratory ship HMS Beagle. During this five-year voyage Darwin observed many species of animals and birds and collected many fossils. His observations on the differences and similarities of species, both living and extinct, led him to ask many questions: Why did some species survive and others die out? Why did certain species live in certain places and not in others? These questions preoccupied him when he returned to England in 1836.

Darwin’s observations led him to doubt the commonly held belief that all the species had been created at once and had remained unchanged through time. The problem was to find out what forces made organisms change. Darwin’s answer was his theory of natural selection: certain members of a species have traits that make them better adapted to their environment. These animals are more successful and therefore have more surviving offspring that inherit these traits. Animals that are not well adapted do not have as many offspring and eventually die out. In this way, species change and certain groups become extinct.

Although Darwin devoted much of his time to his theory of natural selection, he did not publish it for more than 20 years. He knew that his explanation of the species would anger many people, since it did not agree with the dominant Christian theology of the time. Despite early scientific and religious opposition, Darwin’s theory of natural selection is now accepted as the explanation of evolution, at least within the scientific community. However, arguments continue between evolutionists and creationists (those who believe all species were created by God in their present form). Darwin’s theories have indeed changed the way most people view the world, from the evolution of humans to the philosophical bases of science itself.

1865: The birth of genetics. It was unfortunate for the biological sciences that Gregor Mendel was an obscure Austrian monk. His pioneering work in the field of genetics was being done at the time that Charles Darwin’s publications on evolution were beginning to create worldwide controversy, but Mendel’s work would remain unknown for years.

Mendel became an Augustinian monk in 1843, but his abilities in mathematics and the sciences were evident. His experiments on the principles of heredity were begun in about 1856 in what is now Czechoslovakia. By crossing various strains of peas with one another, Mendel found that traits were passed on from generation to generation in what he called “discrete hereditary elements” in sex cells, or gametes.

Mendel reported the results of his experiments to a local society for the study of natural science in 1865 and published his findings in the society’s journal. They were as good as buried there for the next 35 years. Although the journal found its way to libraries in Europe and North America, few paid any attention to his writings. When other botanists obtained results similar to Mendel’s, they began searching through earlier writings on the subject. Only then was Mendel’s 1865 research revealed. His “discrete hereditary elements” are now called genes, and the new science once called Mendelism is known as genetics.

Two Pioneers of Genetics

In 1859 the English biologist Charles Darwin published his epic ‘The Origin of Species’, an attempt to demonstrate that all living things are related through the common bond of evolution. Darwin assumed that all species produce more offspring than reach maturity. Those offspring that survive and reproduce, he reasoned, do so because they are better suited to the existing environment. Because environment changes with time, he argued, species must either adapt to the new conditions or become extinct. Darwin did not know just what mechanisms made it possible for such changes in species to take place. He recognized, however, that if his theory were correct, changeable or mutable units of heredity must exist and that variations in species must arise as a result of an accumulation of small changes in these units of heredity.

In 1865 Gregor Mendel, a monk in an Austrian Roman Catholic monastery, wrote a paper that laid the foundation for modern genetics. Mendel was the first to demonstrate experimentally the manner in which specific traits are passed on from one generation to the next. He concluded that “discrete hereditary elements” (not called genes until the 1900s) in the sex cells are responsible for the transmission of traits. Mendel was ahead of his time, however. The significance of his work was not realized until 1900.

Mendel’s Contributions to Genetics

Pea, a climbing pod-bearing plant (Pisum sativum), or its seed.

In the monastery garden where he conducted his experiments, Mendel observed the inheritance of traits in the easily available garden pea, Pisum sativum. The plant is an ideal genetic working material because a number of progeny can be produced in a short time and because its reproductive parts are so constructed that accidental fertilization is nearly impossible.

Mendel began by tracing the inheritance of one or two contrasting traits at a time. Thus, he crossed tall peas with short peas or red-flowered peas with white-flowered peas. Then he recorded how many of the progeny developed each of the contrasting traits. He used the progeny in subsequent matings to follow the progress of the traits under study through a number of generations.

Somatic cells (or body cells), cells of the body that compose the tissues, organs, and parts of that individual other than the germ cells.

Gamete (or germ cell), sex cell that fuses with a cell of the opposite sex to form new life.

From the evidence obtained in this way, Mendel reasoned that contrasting traits are governed by units of inheritance existing in pairs in somatic, or body, cells but singly in gametes, or sex cells. If the genotype R stands for red and the genotype r for white, then homozygous red-flowered peas have RR somatic cells and R gametes. The somatic cells and gametes of homozygous white-flowered peas are, by contrast, rr and r, respectively.

Allele, in genetics; an alternate form of gene located on a specific site on a chromosome.

The separation of alleles (R from r, for example) in gamete formation is called the principle of segregation. Mendel correctly assumed that chance determines which gene of a pair finds its way into a given gamete. A red-flowered pea may be a heterozygous, or hybrid, Rr. That is, in some way the allele for red flowers (R) “dominates” the allele for white flowers (r). However, the R and r alleles of the hybrid segregate during sex-cell division to produce an equal number of R and r gametes. This is proved by test crossing the hybrid with a homozygous white (rr) plant. Since the homozygous white produces only r gametes and the hybrid produces both R and r gametes, the ratio of red plants to white plants is one to one.

Mendel also demonstrated that non allelic genes (for tall or short and red or white phenotypes, for example) segregate independently of one another into the gametes. This phenomenon is called the principle of independent assortment. For example, a cross between pure strains of tall plants with red flowers (TTRR) and short plants with white flowers (ttrr) produces hybrid progeny that are all tall with red flowers (TtRr). A test cross between these tall, red hybrids (TtRr) and short, white pure strains (ttrr) results in four equally distributed types of progeny 25 percent tall, red TtRr, 25 percent short, red ttRr, 25 percent tall, white Ttrr, and 25 percent short, white ttrr. Modern geneticists have learned, however, that independent assortment does not always hold true because non alleles located side by side on the same chromosome tend to be inherited as a package.

1953: Discovery of DNA structure. The full name of DNA is deoxyribonucleic acid. It carries the codes of genetic information that transmit inherited characteristics to successive generations of living things.

DNA was discovered in 1869 by Friedrich Miescher. In 1943 its role in inheritance was demonstrated. In 1953 its structure was determined by an American biochemist, James D. Watson, and an English physicist, Francis H.C. Crick. Watson and Crick showed the structure to be two strands of a phosphoryl-deoxyribose polymer arranged as a double helix. Watson and Crick were awarded the Nobel prize in physiology or medicine in 1962.

1973: Biotechnology. Two American biochemists, Stanley H. Cohen and Herbert W. Boyer, inaugurated the science of genetic engineering and its associated field of biotechnology in 1973. They showed that it was possible to break down DNA into fragments and combine them into new genes, which could in turn be placed in living cells. There they would reproduce each time a cell divided into two parts.

Genetic engineering makes it possible to modify existing organisms or create organisms that already exist in the human body but that are difficult to isolate. For example, one early product was a genetically engineered form of insulin, used in the treatment of diabetes. Other genetically engineered products include interferons, which are used in the treatment of viral infections and showed promise in the treatment of various forms of cancer. Scientists hope that genetically engineered products will someday prevent or cure such genetic disorders as muscular dystrophy and cystic fibrosis.

Genetic engineering also opens the possibility of creating entirely new organisms. In 1980 the United States Supreme Court ruled that newly developed organisms could be patented, thus giving ownership rights to the companies that made them.

Posted 2012/04/19 by Stelios in Education

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GENETICS (Part 2 of 3)   Leave a comment

Genetic Research After Mendel

Chromosome, microscopic, threadlike part of the cell that carries hereditary information in the form of genes; among simple organisms, such as bacteria and algae, chromosomes consist entirely of DNA and are not enclosed within a membrane; among all other organisms chromosomes are contained in a membrane-bound cell nucleus and consist of both DNA and RNA; arrangement of components in the DNA molecules determines the genetic information; every species has a characteristic number of chromosomes, called the chromosome number; in species that reproduce asexually the chromosome number is the same in all the cells of the organism; among sexually reproducing organisms, each cell except the sex cell contains a pair of each chromosome.

Weismann, August (1834-1914), German biologist; advanced theory that changes in the characteristics of a species are due to changes in germ plasm.

Sutton, Walter S. (1876-1916), U.S. geneticist and physician; noted for studies of chromosomes.

Boveri, Theodor Heinrich (1862-1915), German scientist whose work with roundworm eggs proved that chromosomes are separate, continuous entities within the nucleus of a cell.

Chromosomes, structures in the cell nucleus that carry genes, were discovered after Mendel’s work was published. However, accurate accounts of their behaviour were not generally available until about 1885. Earlier the German biologist August Weismann had suggested that heredity depends on a special material called germ plasma that is transmitted unaltered from one generation to another. In the 1880s Weismann and other scientists advanced the idea that the germ plasm was located in the chromosomes. In 1902 Walter S. Sutton of the United States and Theodor Boveri of Germany independently recognized the connection between the segregation of alleles as described by Mendel and the segregation of homologous pairs of chromosomes in the division of sex cells.

Morgan, Thomas Hunt (1866-1945), U.S. zoologist, born in Lexington, Ky.; professor Columbia University 1904-28; director of biological laboratories, California Institute of Technology; received 1933 Nobel prize for work on role of chromosomes in heredity; wrote books on embryology, evolution, and heredity.

In 1910 the American geneticist Thomas H. Morgan and his associates discovered that genes occur on chromosomes and that those genes lying close together on the same chromosome form linkage groups that tend to be inherited together. They also showed that linkage groups often break apart naturally as a result of a phenomenon called crossing over.

Beadle, George Wells (1903-89), U.S. biologist, born near Wahoo, Neb.; professor and chairman of biology division California Institute of Technology 1946-60, acting dean of faculty 1960-61; president University of Chicago 1961-68; director Institute of Biomedical Research, AMA, 1968-70; received 1958 Nobel prize for work in biochemical and microbial genetics.

Tatum, Edward Lawrie (1909-75), U.S. biochemist, born in Boulder, Colo.; professor Yale University 1946-48, Stanford University 1948-57, and Rockefeller University 1957-75; received 1958 Nobel prize for discovery that genes act by controlling specific chemical processes.

Avery, Oswald Theodore (1877-1955), U.S. bacteriologist who determined that deoxyribonucleic acid (DNA) is the basic genetic material of the cell.

Watson, James Dewey (born 1928), U.S. biochemist, born in Chicago, Ill.; on staff Harvard University 1955-68, professor 1961-68; director Cold Spring Harbor Laboratory from 1968; received 1962 Nobel prize for discovery of molecular structure of DNA.

Crick, Francis Harry Compton (born 1916), British biochemist; on staff Cavendish Laboratory, Cambridge University 1949-77; professor Salk Institute for Biological Studies from 1977; received 1962 Nobel prize for discovery of molecular structure of DNA; elected to U.S. National Academy of Sciences 1969.

Jacob, Francois (born 1920), French biologist, born in Nancy; with Pasteur Institute from 1950, College de France from 1964; received 1965 Nobel prize for work in genetics.

Monod, Jacques (1910-76), French biologist, born in Paris; with Pasteur Institute from 1945, director from 1971; received 1965 Nobel prize for work in genetics; researched protein metabolism and RNA.

In the 1940s George W. Beadle and Edward L. Tatum of the United States began to investigate the role played by genes in the production of enzymes. By 1944 Oswald T. Avery had discovered that deoxyribonucleic acid (DNA) was the basic genetic material of the cell. The precise molecular structure of DNA was determined in 1953 by James D. Watson of the United States and Francis H.C. Crick of England. By 1961 the French geneticists Francois Jacob and Jacques Monod had developed a model for the process by which DNA directs the synthesis of proteins, thereby deciphering, in principle, the genetic code of the DNA molecule. In 1988 an international team of scientists began a project to devise a map of the human genome, all the genes that determine the make-up of a human being.

Recombinant DNA, genetically engineered DNA prepared in vitro by cutting up DNA molecules and splicing together specific DNA fragments; usually uses DNA from more than one species of organism.

Clone, process of biologically purifying a gene from one species by inserting it into the DNA of another species where it is replicated along with the host DNA; used to manufacture insulin.

Since the 1970s the techniques of recombinant DNA have allowed researchers to biologically purify, or clone, a gene from one species by inserting it into the DNA of another species, where it is replicated along with the host DNA. In this manner human hormones, such as insulin and growth hormone, have been manufactured economically by colonies of bacteria.

CHROMOSOMES AND CELL DIVISION

Chromosomes are mainly aggregates of deoxyribonucleic acid (DNA) and protein. All but the simplest kinds of plants and animals inherit two sets of chromosomes (the diploid number), one set (the haploid number) from each parent. In humans, each somatic cell has a haploid set of 23 chromosomes from each parent, for a total of 46.

The chromosomes within each set vary in appearance. However, each has a homologous partner in the other set, which resembles it in both appearance and genetic characteristics. A given gene is found on only a particular chromosome in each set. Its allele is on that chromosome’s homologue in the other set. The alleles are passed on to new cells during mitosis, the division of somatic cells.

Mitosis takes place as soon as a sperm fertilizes an egg. It continues throughout the life of the organism. Prior to mitosis, the cell chromosomes make exact copies of themselves. At this point, twice the diploid number of chromosomes exist in the cell. As mitosis proceeds, one set of the doubled chromosomes goes into each of the two daughter cells. Each thus acquires a full diploid set of chromosomes. This process is repeated again and again as cells divide and the body grows. Sex cells, however, divide in a different way.

Sex cells in the adult reproductive organs produce gametes by meiosis. This process consists of two divisions. As the first division proceeds, the homologous chromosomes in the nucleus of the sex cell seek each other out and join, or synapse. They are called bivalents at this point.

Then the bivalents duplicate themselves to form a bundle, or tetrad, or four intertwined chromatids. The tetrads then thicken and separate, and a pair of homologous chromatids pass into each of two daughter cells.

Meiosis does not stop at this stage, however. The two daughter cells, still with a diploid number of chromosomes, undergo a second division, the reduction division. In this division, the homologous chromatids do not duplicate themselves but merely separate and pass randomly into two additional cells, where they thicken into chromosomes. In meiosis, each sex cell produces four gametes, each with a haploid number of chromosomes (only one allele is in each gamete). When a male gamete fertilizes an egg, the diploid number of chromosomes is restored.

Chromosomes are fully visible under a microscope during the four stages of cell division prophase, metaphase, anaphase, and telophase. However, between the telophase and the next prophase a lengthy period called the interphase occurs, during which the chromosomes are too thin and strung out to be seen. Important chemical activities take place during the interphase. Ribonucleic acid (RNA), chemically related to DNA, and proteins are synthesized during the lengthy interphase as well as during the relatively short period of cell division.

Late in the interphase, DNA is synthesized and daughter chromosomes are created. First, DNA is made. Soon afterwards, in a burst of activity, chromosomal DNA, RNA, and protein are fitted together, the chromosomes begin to take shape, and cell division begins. During sex cell division, however, an important gene exchange between homologous chromosomes takes place.

Linked Non alleles and Crossing Over

As meiosis takes place, homologous chromosomes exchange some of their genes. This phenomenon is known as crossing over. Although the process is not well understood, it is thought that a reciprocal breakage and rejoining of homologous chromatids occurs while the tetrads are intertwined during early meiosis.

Geneticists began to investigate crossing over when they noted that the traits actually inherited did not always adhere to the principle of independent assortment. Test crosses between AaBb and aabb parents A, a, B, and b representing the dominant and recessive genes of non alleles did not always produce equal numbers of AaBb, aaBb, Aabb, and aabb progeny but a greater number of the parental types AaBb and aabb and a smaller number of the recombinant types Aabb and aaBb. Geneticists concluded that the dominant non alleles A and B were linked together on one homologous chromosome and that the recessive non alleles a and b were linked together on the other. If this linkage were unbreakable, in meiosis the hybrid AaBb would form only AB and ab gametes. In fact, however, Ab and aB gametes were also formed the frequency varying for different linked non alleles It was therefore surmised that an exchange, or crossing over, took place.

Posted 2012/04/19 by Stelios in Education

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GENETICS (Part 3 of 3)   Leave a comment

Sex Linkage

Linked genes occur on the sex chromosomes as well as on the non sex chromosomes, or autosomes. In humans, a woman carries two X chromosomes and 44 autosomes in each body cell and one X chromosome and 22 autosomes in each egg. A man carries one X and one Y chromosome and 44 autosomes in each body cell and either an X or a Y chromosome and 22 autosomes in each sperm cell.

Only sons inherit traits carried by genes located on the Y chromosome, because a boy (XY) develops whenever a Y sperm fertilizes an egg. Traits carried on genes located on an X chromosome of the father are transmitted only to daughters (XX).

GENES AND THE GENETIC CODE

Genes, the arbiters of body form and organ function, work with precision. They transmit to each cell a genetic code that determines the cell’s purpose.

Nucleic Acids The Key to Heredity

Nucleic acid, any of substances comprising genetic material of living cells; divided into two classes: RNA (ribonucleic acid) and DNA (deoxyribonucleic acid); directs protein synthesis and is vehicle for transmission of genetic information from parent to offspring.

The structure of DNA makes gene transmission possible. Since genes are segments of DNA, DNA must be able to make exact copies of itself to enable the next generation of cells to receive the same genes.

Adenine, a purine base that codes hereditary information in the genetic code in DNA and RNA.

Cytosine, pyrimidine base that codes genetic information in DNA or RNA.

The DNA molecule looks like a twisted ladder. Each “side” is a chain of alternating phosphate and deoxyribose sugar molecules. The “steps” are formed by bonded pairs of purine-pyrimidine bases. DNA contains four such bases the purines adenine (A) and guanine (G) and the pyrimidines cytosine (C) and thymine (T).

The RNA molecule, markedly similar to DNA, usually consists of a single chain. The RNA chain contains ribose sugars instead of deoxyribose. In RNA, the pyrimidine uracil (U) replaces the thymine of DNA.

DNA and RNA are made up of basic units called nucleotides. In DNA, each of these is composed of a phosphate, a deoxyribose sugar, and either A, T, G, or C. RNA nucleotides consist of a phosphate, a ribose sugar, and either A, U, G, or C.

Nucleotide chains in DNA wind around one another to form a complete twist, or gyre, every ten nucleotides along the molecule. The two chains are held fast by hydrogen bonds linking A to T and C to G A always pairs with T (or with U in RNA); C always pairs with G. Sequences of the paired bases are the foundation of the genetic code. Thus, a portion of a double-stranded DNA molecule might read: A-T C-G G-C T-A G-C C-G A-T. When “unzipped,” the left strand would read: ACGTGCA; the right strand: TGCACGT.

DNA is the “master molecule” of the cell. It directs the synthesis of RNA. When RNA is being transcribed, or copied, from an unzipped segment of DNA, RNA nucleotides temporarily pair their bases with those of the DNA strand. In the preceding example, the left hand portion of DNA would transcribe a strand of RNA with the base sequence: UGCACGU.

Genes and Protein Synthesis

A genetic code guides the assembly of proteins. The code ensures that each protein is built from the proper sequence of amino acids.

Genes transmit their protein-building instructions by transcribing a special type of RNA called messenger RNA (mRNA). This leaves the cell nucleus and moves to structures in the cytoplasm called ribosomes, where protein synthesis takes place.

Cell biologists believe that DNA also builds a type of RNA called transfer RNA (tRNA), which floats freely through the cell cytoplasm. Each tRNA molecule links with a specific amino acid. When needed for protein synthesis, the amino acids are borne by tRNA to a ribosome.

For years biologists wondered how amino acids were guided to fit together in the exact sequences needed to produce the thousands of kinds of proteins required to sustain life. The answer seems to lie in the way the four genetic “code letters” A, T, C, and G are arranged along the DNA molecule.

The Genetic Code

Experimental evidence indicates that the genetic code is a “triplet” code; that is, each series of three nucleotides along the DNA molecule orders where a particular amino acid should be placed in a growing protein molecule. Three-nucleotide units on an mRNA strand for example UUU, UUG, and GUU are called codons. The codons, transcribed from DNA, are strung out in a sequence to form mRNA.

According to the triplet theory, tRNA contains anti codons, nucleotide triplets that pair their bases with mRNA codons. Thus, AAA is the anti codon for UUU. When a codon specifies a particular amino acid during protein synthesis, the tRNA molecule with the anti codon delivers the needed amino acid to the bonding site on the ribosome.

The genetic code consists of 64 codons. However, since these codons order only some 20 amino acids, most, if not all, of the amino acids can be ordered by more than one of them. For example, the mRNA codons UGU and UGC both order cysteine. Because mRNA is a reverse copy of DNA the genetic code for cysteine is ACA or ACG. Some codons may act only to signal a halt to protein synthesis.

To illustrate the operation of the genetic code, assume that one protein is responsible for the development of brown hair and that this protein is composed of three amino acid molecules arranged in linear sequence for example, cysteine-cysteine-cysteine. (This is a much simplified example, since proteins actually incorporate from 100 to 300 amino acid molecules.) The gene (DNA segment) specifying formation of this protein reads: ACAACAACA. It produces the mRNA segment UGUUGUUGU. This segment then drifts to a ribosome. Three tRNA molecules, each with the cysteine-bearing anti codon ACA, line up in order on the ribosome and deposit their cysteine to make the brown-hair protein.

Since code transmission from DNA to mRNA is extremely precise, any error in the code affects protein synthesis. If the error is serious enough, it eventually affects some body trait or feature.

Mutations

Down’s syndrome (or mongolism), a congenital condition with moderate to severe mental retardation; characteristic features include: broad flat faces, slanted eyes, small ears and noses; heart defects and other abnormalities.

Certain chemicals and types of radiation can cause mutations changes in the structure of genes or chromosomes. The simplest type of mutation is a change in the DNA or RNA nucleotide sequence. Mutations may also involve the number of chromosomes or the gain, loss, or rearrangement of chromosome segments. If a mutation occurs in parental sex cells, the change is passed on to the offspring. In humans, an extra chromosome in body cells (47 instead of 46) has been implicated in Down’s syndrome, a serious mental abnormality.

Most mutations are considered harmful and are, therefore, eventually eliminated. Some, however, enable an organism to adapt to a changing environment. Biologists believe that mutations have caused the many genetic changes involved in the evolution of species.

Assisted by Val W. Woodward

Genetic Terms

allele. One of the members of a gene pair, each of which is found on chromosomes; the pair of alleles determines a specific trait.

chromosome. A structure in the cell nucleus containing genes.

dominance. The expression of one member of an allelic pair at the expense of the other in the phenotypes of heterozygotes.

gene. One of the chromosomal units that transmit specific hereditary traits; a segment of the self-reproducing molecule, deoxyribonucleic acid.

genotype. The genetic make-up of an organism, which may include genes for the traits that do not show up in the phenotype.

heterozygous. Containing dissimilar alleles.

homozygous. Containing a pair of identical alleles.

phenotype. The visible characteristics of an organism (for example, height and colouration).

recessiveness. The masking of one member of an allelic pair by the other in the phenotypes of heterozygotes.

Posted 2012/04/19 by Stelios in Education

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PASSOVER   Leave a comment

DEFINITION: a Jewish holiday (Pesach ) celebrated for eight (or seven) days beginning on the 14th of Nisan and commemorating the deliverance of the ancient Hebrews from slavery in Egypt.

One of the major festivals in Judaism is Passover. It is a holiday of rejoicing when Jews all over the world recall their deliverance from slavery in Egypt. The word Passover comes from the idea that God passed over the houses of the Israelites, who had marked their door posts to signify that they were children of God. This way the first born sons of the Jews were spared when God smote the first born sons of the Egyptian taskmasters on the eve of the Exodus.

Passover is celebrated each spring for eight days beginning on Nisan 15 of the Hebrew calendar. Families gather at the beginning of Passover for the Seder meal. The meal is preceded by prayers and songs from the Haggadah, the narration of the events surrounding the Exodus from Egypt. All of the foods eaten are symbolic. These include bitter herbs, reminiscent of the pain of bondage; a roasted lamb bone to recall offerings that the Israelites made to God; unleavened bread called matzo, which is eaten all week instead of leavened bread because the Israelites lacked time even for dough to rise in their haste to escape from Egypt; and a tasty mixture of nuts, apples, honey, and wine to symbolize the mortar the Jewish slaves were forced to use to build Egyptian temples.

During the Seder it is traditional for the youngest child to ask four questions about the uniqueness of Passover, which the leader answers. Children are encouraged to participate and to think of their history as if they themselves had been delivered from slavery. They are also taught in the Haggadah that, because the Israelites were strangers in Egypt, Jews must remember to welcome strangers in their midst.

Posted 2012/04/16 by Stelios in Education

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